diff --git a/src/Functional/functions.php b/src/Functional/functions.php
index 3030903..bfd10c0 100644
--- a/src/Functional/functions.php
+++ b/src/Functional/functions.php
@@ -26,7 +26,7 @@
  *
  * @return mixed
  */
-function applicator($x, callable $f = null)
+function applicator($x, ?callable $f = null)
 {
     return curryN(2, function ($y, callable $f) {
         return $f($y);
@@ -168,7 +168,7 @@ function reverse(callable $function)
  * @param callable $transformation
  * @param Functor  $value
  */
-function map(callable $transformation, Functor $value = null)
+function map(callable $transformation, ?Functor $value = null)
 {
     return curryN(2, function (callable $transformation, Functor $value) {
         return $value->map($transformation);
@@ -188,7 +188,7 @@ function map(callable $transformation, Functor $value = null)
  * @param callable $function
  * @param Monad    $value
  */
-function bind(callable $function, Monad $value = null)
+function bind(callable $function, ?Monad $value = null)
 {
     return curryN(2, function (callable $function, Monad $value) {
         return $value->bind($function);
@@ -226,7 +226,7 @@ function join(Monad $monad): Monad
  *
  * @return mixed
  */
-function reduce(callable $callable, $accumulator = null, Foldable $foldable = null)
+function reduce(callable $callable, $accumulator = null, ?Foldable $foldable = null)
 {
     return curryN(3, function (
         callable $callable,
@@ -254,7 +254,7 @@ function reduce(callable $callable, $accumulator = null, Foldable $foldable = nu
  *
  * @return mixed
  */
-function foldr(callable $callable, $accumulator = null, Foldable $foldable = null)
+function foldr(callable $callable, $accumulator = null, ?Foldable $foldable = null)
 {
     return curryN(3, function (
         callable $callable,
@@ -282,7 +282,7 @@ function foldr(callable $callable, $accumulator = null, Foldable $foldable = nul
  *
  * @return Foldable|\Closure
  */
-function filter(callable $predicate, Foldable $list = null)
+function filter(callable $predicate, ?Foldable $list = null)
 {
     return curryN(2, function (callable $predicate, Foldable $list) {
         return reduce(function (Listt $list, $x) use ($predicate) {
@@ -394,9 +394,9 @@ function reThrow(\Exception $e)
  * @return Monad|\Closure
  */
 function liftM2(
-    callable $transformation = null,
-    Monad $ma = null,
-    Monad $mb = null
+    ?callable $transformation = null,
+    ?Monad $ma = null,
+    ?Monad $mb = null
 ) {
     return call_user_func_array(
         liftA2,
@@ -419,9 +419,9 @@ function liftM2(
  * @return Monad|\Closure
  */
 function bindM2(
-    callable $transformation = null,
-    Monad $ma = null,
-    Monad $mb = null
+    ?callable $transformation = null,
+    ?Monad $ma = null,
+    ?Monad $mb = null
 ) {
     return curryN(
         3,
@@ -454,9 +454,9 @@ function (
  * @return Applicative|\Closure
  */
 function liftA2(
-    callable $transformation = null,
-    Applicative $fa = null,
-    Applicative $fb = null
+    ?callable $transformation = null,
+    ?Applicative $fa = null,
+    ?Applicative $fb = null
 ) {
     return curryN(3, function (
         callable $transformation,
@@ -491,7 +491,7 @@ function liftA2(
  *
  * @return Monad|\Closure
  */
-function sequenceM(Monad $a, Monad $b = null): Monad
+function sequenceM(Monad $a, ?Monad $b = null): Monad
 {
     return curryN(2, function (Monad ...$monads): Monad {
         return array_reduce($monads, function (?Monad $a, Monad $b) {
@@ -519,7 +519,7 @@ function sequenceM(Monad $a, Monad $b = null): Monad
  *
  * @return \Closure|Applicative f (t b)
  */
-function traverse(callable $transformation, Traversable $t = null)
+function traverse(callable $transformation, ?Traversable $t = null)
 {
     return curryN(2, function (
         callable $transformation,
@@ -542,7 +542,7 @@ function traverse(callable $transformation, Traversable $t = null)
  *
  * @return \Closure|Monad m [a]
  */
-function filterM(callable $f, Foldable $xs = null)
+function filterM(callable $f, ?Foldable $xs = null)
 {
     return curryN(2, function (
         callable $f,
@@ -585,7 +585,7 @@ function filterM(callable $f, Foldable $xs = null)
  *
  * @return mixed m a
  */
-function foldM(callable $f, $z0 = null, Foldable $xs = null)
+function foldM(callable $f, $z0 = null, ?Foldable $xs = null)
 {
     return curryN(3, function (
         callable $f,
diff --git a/src/Functional/listt.php b/src/Functional/listt.php
index 431b609..7146106 100644
--- a/src/Functional/listt.php
+++ b/src/Functional/listt.php
@@ -132,7 +132,7 @@ function concat(Foldable $xs): Listt
  * @param  Listt          $xs
  * @return Listt|\Closure
  */
-function prepend($x, Listt $xs = null)
+function prepend($x, ?Listt $xs = null)
 {
     return curryN(2, function ($x, Listt $xs): Listt {
         return new ListtCons(function () use ($x, $xs) {
@@ -160,7 +160,7 @@ function prepend($x, Listt $xs = null)
  * @param  Listt|null     $b
  * @return Listt|callable
  */
-function append(Listt $a, Listt $b = null)
+function append(Listt $a, ?Listt $b = null)
 {
     return curryN(2, function (Listt $a, Listt $b): Listt {
         return $a->concat($b);
diff --git a/src/Functional/monoid.php b/src/Functional/monoid.php
index 2c54146..59b90cd 100644
--- a/src/Functional/monoid.php
+++ b/src/Functional/monoid.php
@@ -31,7 +31,7 @@ function emptyM(Monoid $a): Monoid
  *
  * @return Semigroup|\Closure
  */
-function concatM(Semigroup $a, Semigroup $b = null)
+function concatM(Semigroup $a, ?Semigroup $b = null)
 {
     return curryN(2, function (Semigroup $a, Semigroup $b) {
         return $a->concat($b);
diff --git a/src/Functional/predicates.php b/src/Functional/predicates.php
index ef6b411..bb3c015 100644
--- a/src/Functional/predicates.php
+++ b/src/Functional/predicates.php
@@ -50,7 +50,7 @@ function lt($expected, $value = null)
  *
  * @return bool|\Closure
  */
-function orr(callable $predicateA, callable $predicateB = null, $value = null)
+function orr(callable $predicateA, ?callable $predicateB = null, $value = null)
 {
     return curryN(3, function (callable $a, callable $b, $value): bool {
         return $a($value) || $b($value);
diff --git a/src/Functional/setoid.php b/src/Functional/setoid.php
index 5bc3a38..b07bd79 100644
--- a/src/Functional/setoid.php
+++ b/src/Functional/setoid.php
@@ -16,7 +16,7 @@
  *
  * @return bool|\Closure
  */
-function equal(Setoid $a, Setoid $b = null)
+function equal(Setoid $a, ?Setoid $b = null)
 {
     return curryN(2, function (Setoid $a, Setoid $b):bool {
         return $a->equals($b);
diff --git a/src/Functional/sublist.php b/src/Functional/sublist.php
index 8b0eae0..eb1faec 100644
--- a/src/Functional/sublist.php
+++ b/src/Functional/sublist.php
@@ -23,7 +23,7 @@
  * @param  Listt          $xs
  * @return Listt|\Closure
  */
-function take(int $n, Listt $xs = null)
+function take(int $n, ?Listt $xs = null)
 {
     return curryN(2, function (int $n, Listt $xs): Listt {
         if ($n < 1) {
@@ -49,7 +49,7 @@ function take(int $n, Listt $xs = null)
  * @param  Listt          $xs
  * @return Listt|\Closure
  */
-function drop(int $n, Listt $xs = null)
+function drop(int $n, ?Listt $xs = null)
 {
     return curryN(2, function (int $n, Listt $xs): Listt {
         if ($n < 1) {
@@ -84,7 +84,7 @@ function drop(int $n, Listt $xs = null)
  * @param  Listt          $xs
  * @return Listt|\Closure
  */
-function dropWhile(callable $predicate, Listt $xs = null)
+function dropWhile(callable $predicate, ?Listt $xs = null)
 {
     return curryN(2, function (callable $predicate, Listt $xs): Listt {
         if ($xs instanceof ListtNil) {
@@ -126,7 +126,7 @@ function dropWhile(callable $predicate, Listt $xs = null)
  * @param  Listt          $xs
  * @return array|\Closure
  */
-function span(callable $predicate, Listt $xs = null)
+function span(callable $predicate, ?Listt $xs = null)
 {
     return curryN(2, function (callable $predicate, Listt $xs): array {
         try {
diff --git a/src/Functional/zipping.php b/src/Functional/zipping.php
index 4b3d405..8f46496 100644
--- a/src/Functional/zipping.php
+++ b/src/Functional/zipping.php
@@ -23,7 +23,7 @@
  * @param  Listt|null     $ys
  * @return Listt|\Closure
  */
-function zip(Listt $xs, Listt $ys = null)
+function zip(Listt $xs, ?Listt $ys = null)
 {
     return curryN(2, function (Listt $xs, Listt $ys): Listt {
         try {
diff --git a/src/Monad/Either/functions.php b/src/Monad/Either/functions.php
index 5c2cbd1..460df68 100644
--- a/src/Monad/Either/functions.php
+++ b/src/Monad/Either/functions.php
@@ -62,7 +62,7 @@ function left($value)
  *
  * @return mixed c
  */
-function either(callable $left, callable $right = null, Either $either = null)
+function either(callable $left, ?callable $right = null, ?Either $either = null)
 {
     return f\curryN(3, function (callable $left, callable $right, Either $either) {
         return $either->either($left, $right);
@@ -82,7 +82,7 @@ function either(callable $left, callable $right = null, Either $either = null)
  *
  * @return Left|Right|\Closure
  */
-function doubleMap(callable $left, callable $right = null, Either $either = null)
+function doubleMap(callable $left, ?callable $right = null, ?Either $either = null)
 {
     return f\curryN(3, function (callable $left, callable $right, Either $either) {
         return either(
@@ -106,7 +106,7 @@ function doubleMap(callable $left, callable $right = null, Either $either = null
  *
  * @return Either|\Closure
  */
-function tryCatch(callable $function = null, callable $catchFunction = null, $value = null)
+function tryCatch(?callable $function = null, ?callable $catchFunction = null, $value = null)
 {
     return f\curryN(3, function (callable $function, callable $catchFunction, $value) {
         return f\tryCatch(
@@ -140,9 +140,9 @@ function toMaybe(Either $either)
  * @param  Either $either
  * @return mixed
  */
-function fromLeft($a, Either $either = null)
+function fromLeft($a, ?Either $either = null)
 {
-    return f\curryN(2, function ($a, Either $either = null) {
+    return f\curryN(2, function ($a, ?Either $either = null) {
         return either(f\identity, f\constt($a), $either);
     })(...func_get_args());
 }
@@ -156,9 +156,9 @@ function fromLeft($a, Either $either = null)
  * @param  Either $either
  * @return mixed
  */
-function fromRight($a, Either $either = null)
+function fromRight($a, ?Either $either = null)
 {
-    return f\curryN(2, function ($a, Either $either = null) {
+    return f\curryN(2, function ($a, ?Either $either = null) {
         return either(f\constt($a), f\identity, $either);
     })(...func_get_args());
 }
diff --git a/src/Monad/Free/functions.php b/src/Monad/Free/functions.php
index 3223793..049810a 100644
--- a/src/Monad/Free/functions.php
+++ b/src/Monad/Free/functions.php
@@ -39,7 +39,7 @@ function liftF(Functor $f): MonadFree
  *
  * @return Monad|callable
  */
-function foldFree(callable $interpreter, MonadFree $free = null, callable $return = null)
+function foldFree(callable $interpreter, ?MonadFree $free = null, ?callable $return = null)
 {
     return curryN(3, function (callable $interpreter, MonadFree $free, callable $return): Monad {
         return $free->foldFree($interpreter, $return);
diff --git a/src/Monad/IO/errors.php b/src/Monad/IO/errors.php
index 21b7192..9cc76e7 100644
--- a/src/Monad/IO/errors.php
+++ b/src/Monad/IO/errors.php
@@ -50,7 +50,7 @@ function throwIO(\Exception $e)
  *
  * @return M\IO|\Closure
  */
-function tryCatch(M\IO $io = null, callable $catchFunction = null)
+function tryCatch(?M\IO $io = null, ?callable $catchFunction = null)
 {
     return f\curryN(2, function (M\IO $io, callable $catchFunction) {
         return M\IO::of(function () use ($io, $catchFunction) {
diff --git a/src/Monad/Maybe/functions.php b/src/Monad/Maybe/functions.php
index 54752d4..11ef56f 100644
--- a/src/Monad/Maybe/functions.php
+++ b/src/Monad/Maybe/functions.php
@@ -53,7 +53,7 @@ function just($value)
  *
  * @return mixed|\Closure
  */
-function maybe($default, callable $fn = null, Maybe $maybe = null)
+function maybe($default, ?callable $fn = null, ?Maybe $maybe = null)
 {
     return f\curryN(3, function ($default, callable $fn, Maybe $maybe) {
         if ($maybe instanceof Nothing) {
@@ -94,7 +94,7 @@ function maybeNull($value = null)
  *
  * @return mixed
  */
-function fromMaybe($default = null, Maybe $maybe = null)
+function fromMaybe($default = null, ?Maybe $maybe = null)
 {
     return f\curryN(2, function ($default, Maybe $maybe) {
         return maybe($default, f\identity, $maybe);
diff --git a/src/Monad/Writer.php b/src/Monad/Writer.php
index 4df6236..244eee6 100644
--- a/src/Monad/Writer.php
+++ b/src/Monad/Writer.php
@@ -11,7 +11,7 @@ class Writer implements FantasyLand\Monad
 {
     const of = 'Widmogrod\Monad\Writer::of';
 
-    public static function of($value, FantasyLand\Monoid $side = null)
+    public static function of($value, ?FantasyLand\Monoid $side = null)
     {
         return new static($value, $side === null ? S::mempty() : $side);
     }
diff --git a/test/Functional/LiftM2Test.php b/test/Functional/LiftM2Test.php
index ab5f85d..2a18425 100644
--- a/test/Functional/LiftM2Test.php
+++ b/test/Functional/LiftM2Test.php
@@ -21,7 +21,7 @@ public function test_it_should_lift2M(
         Monad $mb,
         callable $transformation,
         string $expectedFQCN,
-        callable $valueAssertion = null
+        ?callable $valueAssertion = null
     ) {
         $mc = f\liftM2($transformation, $ma, $mb);